[clang] Adjust modulemap to mark mm3dnow as textual header. (PR #107155)

[James Y Knight via cfe-commits]

jyknight wrote:

The only impact is that any modular header which includes this file will end up with a definition of the header guard, instead of importing it, right? It's not clear to me why it's useful to care about that?

(I totally do understand worrying about cases which result in e.g. having to merge 100s of duplicate function decls -- that can have noticeable performance impact.)

https://github.com/llvm/llvm-project/pull/107155